WO2015145119A1

WO2015145119A1 - Display system

Info

Publication number: WO2015145119A1
Application number: PCT/GB2015/050850
Authority: WO
Inventors: David Grey; Sumanta Talukdar
Original assignee: Wave Optics Ltd
Current assignee: Wave Optics Ltd
Priority date: 2014-03-24
Filing date: 2015-03-23
Publication date: 2015-10-01
Anticipated expiration: 2016-09-24

Abstract

An augmented reality display is disclosed. A projector (11) is provided for creating a collimated input image. Light can be transmitted into a waveguide (12) in which it can travel by total internal reflection. The light is coupled out of the waveguide (12) by a diffraction grating (14) which diffracts light away from a viewer (15). A partially reflective, deformable mirror (18) is provided on the other side of the waveguide (12) from the viewer (15). The mirror (18) is generally arranged to adopt a spherical or parabolic shape and the radius of curvature can be varied so that the mirror (18) can alter the perceived position of the focal plane (17) for the projected light. The mirror (18) reflects light at equal angles, independently of its wavelength. Thus, the optical system and the mirror (18) is able to provide an adjustable focal plane (17) for projected light without introducing any chromatic aberration effects.

Description

Display System

The invention relates to an augmented reality display or a head-up display. These displays allow a user to view their surroundings as well as projected images. In military or transportation applications the projected images can be overlaid on the real world perceived by the user. Other applications for these displays include video games and wearable devices, such as glasses.

In order to provide a constructive augmented reality experience it becomes necessary to adjust the plane of focus of the overlaid display image so that it coincides with, or is close to, the plane of focus of an object of interest in the physical world. For example, if an external object of interest is 3 metres away and the overlaid display image is focused at 30 metres, the viewer will have to switch focus between the two. This can cause eyestrain for the viewer, leading to headaches and discomfort.

Some devices can provide a stereoscopic display for the user, such that a slightly different image is provided to left and right eyes. These displays can provide three- dimensional depth cues in the augmented reality image, based on convergence. However, in these systems the focal plane is often fixed, meaning that the viewer does not have a true three-dimensional experience. Some users also report headaches after using such displays.

One system for addressing these issues is described in WO2012/078410, and Figure 1 shows a system embodying a similar operational principle. As shown in Figure 1 , a projector 1 is provided for creating a collimated input image. The projector 1 is arranged to project light into a glass waveguide 2. Light from the projector is coupled into the waveguide by a diffraction grating 3. The light then travels within the waveguide 2 by total internal reflection. The light is coupled out of the waveguide 2 by another diffraction grating 4 so that it can be viewed by a user 5. The gratings 3, 4 are arranged to operate in reflection. Thus, the first grating 3 is provided on the opposite surface of the waveguide 2 to the surface through which input light is projected. The second grating 4 is provided on the opposite surface of the waveguide 2 to the first grating 3. In this way, the second grating 4 can reflectively diffract light in order to couple it out of the waveguide 2 and towards the viewer 5.

A concave lens 6 with a variable focal length is provided between the waveguide 2 and the viewer 5. The focal length of the concave lens 6 can be adjusted in order to control the perceived position of the focal plane 7 of the projected light.

In Figure 1 , the purpose of the concave lens 6 is to adjust the position of the focal plane 7 of the projected light. However, light from the physical world will also pass through the concave lens 6, and therefore an additional lens 8 is required to compensate its distortion effects. Specifically, a convex lens 8 with a variable focal length is provided on the opposite side of the waveguide 2 to the concave lens 6. The focal length of the convex lens 8 is adjusted synchronously with the concave lens 6 so that there is a neutral effect on light from objects in the real world.

The lenses 6, 8 in Figure 1 typically introduce chromatic aberration effects, since the angles of refraction are dependent on the wavelength of light. These effects require correction through the use of additional lenses (not shown). This leads to a heavy, complex and costly optical system.

An object of the present invention is to provide an improved augmented reality display which permits adjustment of the focal plane of projected light.

According to an aspect of the present invention there is provided a display system for use in an augmented reality display, the system comprising: a projector arranged to project light for augmenting a viewer's perception of a scene; a mirror; and a coupler arranged to receive light from the projector and direct it towards the mirror; wherein the mirror is configured to receive light from the coupler and reflect it towards the viewer, and wherein the mirror is deformable in order to adjust the position of the perceived focal plane of the reflected light.

In this way the display system can provide augmented reality with a changeable virtual conjugate distance, at low cost and without introducing any chromatic aberration. This is possible because the position of the perceived focal plane can be adjusted by changing the reflection conditions at the mirror. The angles of reflection are the same for all wavelengths so that no chromatic aberration is introduced.

Preferably the mirror is at least partially transmissive for light received from the scene. Thus, light received from the scene is preferably not impeded by the presence of the mirror. The augmented reality display may be used to overlay light from the projector on light received from real-world objects in the scene. The mirror may also be only partially reflective for light received from the coupler. The coupler is preferably configured to direct light away from the viewer. Thus, the coupler may re-direct light from the projector in a direction that is along the viewer's line of sight, but away from the viewer. In an alternative the coupler may be arranged to direct light tangentially to the viewer. The system may include a waveguide for receiving light from the projector, such that projected light can advance in the waveguide by total internal reflection. A waveguide may be desirable in a head-up display system. In this way the projector can be provided to one side of the viewer's field of view, and the projected light can be carried by the waveguide into the desired position in the field of view.

The coupler may be a diffraction grating for coupling light out of the waveguide and towards the mirror. Preferably light is coupled in a direction that is along the viewer's line of sight, but away from the viewer. The coupler may be a beam splitter which can receive light from the projector and couple it towards the mirror. The beam splitter may be reflective or transmissive for light from the projector, depending on the operational set-up. A beam splitter may be useful in a free-air augmented reality system. The projector may be arranged to input polarised light and the coupler may be polarisation sensitive. In this way the coupler can process light from the projector and towards the mirror with high efficiency. Where the coupler is a beam splitter it may be arranged to operate with near 100% efficiency if all of the received light has a particular polarisation. A waveplate may be positioned between the coupler and the mirror, for transforming the polarisation of light that passes through it. Preferably still, light may be directed through a quarter-wave plate once by the coupler and once by the mirror, so that linearly polarised light from the projector experiences a 90 degree shift in polarisation angle. In this way, when light is received back at the coupler from the mirror it may have a rotated angle of polarisation. Preferably the effect of the coupler on light received from the mirror is opposite to its effect on light received from the projector. Thus, if the coupler is initially reflective then it may be transmissive following polarisation modification, and vice-versa.

Preferably the display system includes a controller for sending instructions to the deformable mirror. The controller may be under direct control of the viewer so that they can adjust the perceived focal plane of the projected light. In this way the user may be able to adjust the perceived position of the focal plane until it is in a comfortable position.

A camera system may be provided for determining a position of at least one object in the scene, and the controller may be configured to instruct the deformable mirror to adjust the position of the perceived focal plane of the reflected light so that it is approximately equal to the position determined by the camera system. In this way the system may automatically adjust the position of the focal plane of the projected light based on the distance from the viewer to an object in the scene. The camera system may be stereoscopic so that it can determine the object's distance using geometry.

According to another aspect of the present invention a method is provided for operating an optical system in order to adjust the position of the perceived focal plane of projected light, wherein the optical system comprises a projector arranged to project light for augmenting a viewer's perception of a scene; a coupler arranged to receive light from the projector; and a mirror configured to receive light from the coupler and reflect it towards the viewer, wherein the method comprises the step of deforming the mirror in order to adjust the position of the perceived focal plane of the reflected light. According to yet another aspect of the present invention there is provided an optical device, for use in see-through (or non-see through) head up or head mounted displays, comprising: of a partially reflective, deformable, membrane, characterised in that: the said device produces a variable distance, virtual image from a collimated or near collimated ray bundle.

Preferably the curvature of the membrane can be adjusted in real-time to give the user a perceived variation in depth of the image, hence a three-dimensional display. The curvature of the membrane may be adjusted or offset to compensate for the variation in the user's eye accommodation. In addition, polarisation control elements may be incorporated into the device so as to maximise the throughput of light to the eyes. Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram showing a side view of a known augmented reality display;

Figure 2 is a schematic diagram showing a side view of an augmented reality display in an embodiment of the invention;

Figure 3 is a schematic diagram showing a side view of an augmented reality display in another embodiment of the invention; and

Figure 4 is a schematic diagram showing a side view of an augmented reality display in another embodiment of the invention. As is apparent from Figure 2, a projector 1 1 is provided for creating a collimated input image. The projector 1 1 is arranged to project light through a first surface of a glass waveguide 12. Light from the projector is coupled into the waveguide by a diffraction grating 13, which is provided on a second surface of the waveguide 12. The diffraction grating 13 is arranged to operate reflectively in order to optimise efficiency. Light can travel within the waveguide 12 by total internal reflection. The light is coupled out of the waveguide 12 by a second diffraction grating 14, which is also located on the second surface of the waveguide 12. The second diffraction grating 14 is arranged to diffract light away from a viewer 15.

A partially reflective, deformable mirror 18 is provided on the other side of the waveguide 12 from the viewer 15. A controller 19 is connected to the mirror 18, and can control the mirror's deformations. The mirror 18 is generally arranged to adopt a spherical or parabolic shape and the radius of curvature can be varied so that the mirror 18 can alter the perceived position of the focal plane 17 for the projected light. Light that is reflected from the mirror 18 passes through the waveguide 12 to the viewer 15.

The mirror 18 is partially reflective for light received from the waveguide 12. The mirror is also (at least partially) transmissive for light received externally. In this way, a viewer 15 can look through the waveguide 12 and the mirror 18 to view objects in the physical world. The projected light can be overlaid on the physical world, as required, in order to provide an augmented reality experience. The mirror 18 reflects light at equal angles, independently of its wavelength. Thus, the optical system and the mirror 18 is able to provide an adjustable focal plane 17 for projected light without introducing any chromatic aberration effects.

The partially reflective, deformable mirror 18 is preferably controlled by MEMs (microelectromechanical) components. Alternatively, the mirror 18 may be deformed using piezo-electric or air-pressure transducers. These components can adjust the topology of the mirror in order to control its radius of curvature. Deformable mirrors such as these have been used in astronomy for counteracting effects of atmospheric perturbations. The mirror is typically thin (preferably less than 30 micrometres), which means that it does not distort the outside image; in addition, ghosting or secondary images can also be eliminated. The low mass of the mirror means that changes in curvature can be effected quickly. A number of control mechanisms are possible for the mirror 18, within the scope of the present disclosure. In one example, the controller 19 may be linked to an interface (not shown) such as a control pad. In this way the viewer 15 may be able to manipulate the control pad in order to adjust the curvature of the mirror 18. This can allow the viewer 15 to control the position of the perceived focal plane 17 of the projected light. Alternatively, or in addition, the optical system may include forward- looking stereoscopic cameras 21 that can survey the scene. The stereoscopic cameras 21 may be configured to determine the distance of an object in the scene from the viewer 15. The controller 19 can then adjust the curvature of the mirror 18 so that the focal plane 17 matches the distance of the object from the viewer 15. In this way, the focal plane 17 of the projected light can match the distance of the object in the scene so that the viewer 15 does not have to switch focus.

Figure 3 shows another embodiment of the present invention. In this embodiment a projector is arranged to provide a linearly polarised, collimated light beam 21 in a vertical direction. The collimated light beam 21 is arranged to propagate in free space towards a polarisation-sensitive beam splitter 24. The beam splitter 24 is arranged to selectively reflect polarised light in the orientation transmitted by the projector. Thus, the beam splitter can reflect linearly polarised light from the projector with high efficiency. The beam splitter 24 reflects the projected light away from a viewer 25 and towards a deformable mirror 28. The deformable mirror is arranged to reflect the received light back towards the viewer 25. The radius of curvature of the mirror 28 can be adjusted to vary the position of the perceived focal plane 27 of the projected light.

A quarter-wave plate 29 is provided between the beam splitter 24 and the mirror 28. The quarter-wave plate 29 is arranged to rotate linearly polarised light by 45°. The projected light passes through the quarter-wave plate 29 twice following respective reflections by the beam splitter 24 and the mirror 28. Thus, when reflected light is received back at the beam splitter 24 it has been rotated by 90°. The polarisation- sensitive beam splitter 24 is transmissive for polarised light in this orientation, which means that the reflected light can be provided to the viewer 25 with high efficiency. Figure 4 shows another embodiment of the invention. In this embodiment a projector is arranged to provide a linearly polarised, collimated light beam 31 in a horizontal direction. A polarisation-sensitive beam splitter 34 is arranged to selectively transmit polarised light in the orientation transmitted by the projector. Thus, the beam splitter 34 can transmit linearly polarised light from the projector with high efficiency. Light travels from the beam splitter 34 towards a deformable mirror 38, where it is reflected. The deformable mirror is arranged to reflect the received light back towards the beam splitter 34. The radius of curvature of the mirror 38 can be adjusted to vary the position of the perceived focal plane 37 of the projected light.

A quarter-wave plate 39 is provided between the beam splitter 34 and the mirror 38. Thus, when reflected light is received back at the beam splitter 34 it has been rotated by 90°. The polarisation-sensitive beam splitter 34 is reflective for polarised light in this orientation, which means that the reflected light can be provided to the viewer 35 with high efficiency.

The embodiment shown in Figure 4 may be preferred in some circumstances because it can allow the deformable mirror 38 to be provided to the side of a viewer's field of view. This may be preferred for mounting the mirror 38 effectively, and to ensure that the mirror 38 does not interfere with light received from real-world objects in the scene.

Claims

1 . A display system for use in an augmented reality display, the system comprising: a projector arranged to project light for augmenting a viewer's perception of a scene;

a mirror; and

a coupler arranged to receive light from the projector and direct it towards the mirror;

wherein the mirror is configured to receive light from the coupler and reflect it towards the viewer, and wherein the mirror is deformable in order to adjust the position of the perceived focal plane of the reflected light.

2. The display system of claim 1 wherein the mirror is at least partially transmissive for light received from the scene.

3. The display system of claim 1 or claim 2 further comprising a waveguide for receiving light from the projector, such that projected light can advance in the waveguide by total internal reflection.

4. The display system of claim 3 wherein the coupler is a diffraction grating for coupling light out of the waveguide and towards the mirror.

5. The display system of claim 1 or claim 2 wherein the coupler is a beam splitter which can receive light from the projector and couple it towards the mirror.

6. The display system of any of the preceding claims wherein the projector is arranged to input polarised light and the coupler is polarisation sensitive.

7 The display system of claim 6 further comprising a waveplate, positioned between the coupler and the mirror, for transforming the polarisation of light that passes through it.

8. The display system of claim 7 wherein the waveplate is a quarter-wave plate and light is directed through the quarter-wave plate once by the coupler and once by the mirror, so that linearly polarised light from the projector experiences a 90 degree shift in polarisation angle.

9. The display system of any of the preceding claims further comprising a controller for sending instructions to the deformable mirror.

10. The display system of claim 9 further comprising a camera system for determining a position of at least one object in the scene, and wherein the controller is configured to instruct the deformable mirror to adjust the position of the perceived focal plane of the reflected light so that it is approximately equal to the position determined by the camera system.

1 1 . A method of operating a display system for use in an augmented reality display, wherein the display system comprises a projector arranged to project light for augmenting a viewer's perception of a scene; a coupler arranged to receive light from the projector; and a mirror configured to receive light from the coupler and reflect it towards the viewer, wherein the method comprises the step of deforming the mirror in order to adjust the position of the perceived focal plane of the reflected light.